Progress to date has indicated long-range dispersal is not a significant factor in the generation of cyclic fluctuations of populations of the voles Microtus ochrogaster and M. pennsylvanicus. Intrapopulation social interactions appear to form a basis for demographic changes that result in changes in population density sufficient to generate a population cycle. The study is now emphasizing an analysis of social and breeding structure within populations of the two voles. Special attention is being paid to those factors affecting reproduction and juvenile survival (i.e., adult-juvenile interactions, pheromone production, availability and dispersion of optimal food resources, daily activity patterns in relation to food resource characters, and nest predation resulting from behavioral changes in the parents). Field studies include monitoring social structure by use of radiotelemetry and live trapping of a variety of populations of each species; manipulations of species presence and food availability are being conducted in some areas. Confined populations and laboratory studies are also being conducted to analyze the method(s) by which individuals "communicate" social status and cause suppression or acceleration of reproductive maturity.